Stability is among the most fundamental requirements in vehicle design. The footprint of a vehicle must be large enough to prevent significant imbalance and avoid tipping. In particular, personal vehicles such as wheelchairs must be properly designed to prevent the user from falling out of the chair. A sufficient stability margin must be maintained despite shifts in the mass centroid of a patient's body. Patients who are unable to sit up in the middle of the chair tend to lean on the arm rests, shifting the body centroid sideways. When transferring to and from the wheelchair, patients must often undergo unstable transitions, which may cause the wheelchair to fall.
Most vehicles and, therefore, their footprints must conform to dimensional constraints. In particular, for wheelchairs in residential use, vehicle width, sometimes referred to as the tread, must be narrower than the standard doorway. All commercially available wheelchairs are restricted to this doorway dimension which is only 700 mm.
Vehicles designed for improved maneuverability improvement sometimes sacrifice stability and safety. For such designs, the footprint size will be minimized since a large footprint degrades precise movements especially within a closely confined place. This is a particular problem with residential wheelchairs in rooms such as bathrooms. Traditional vehicle designs based on fixed footprints are inefficient in solving the stability-maneuverability trade-off problem.
A holonomic vehicle does not need to change the direction of its wheels to change its direction of motion or rotation. The term "holonomic" means capable of arbitrary motion in an arbitrary direction on a planar surface from an arbitrary initial position and configuration. An omnidirectional personal vehicle using aspects of this design is described in a pending U.S. patent application filed on Apr. 21, 1997 with Ser. No. 08/840,522. This application as well as the associated references cited within this application are herein incorporated by reference. The vehicle has at least three ball wheels, each controlled by an independent motor or actuator, each ball wheel generating a traction force in a different direction in the plane of the surface over which the vehicle moves. The resultant force acting on the vehicle is given by the vectorial sum of the traction forces. Varying the combination of the traction forces creates a specified motion of the vehicle. There is no singular point in this system, hence it is omnidirectional and holonomic. Moreover, this ball wheel vehicle allows for smooth motion with no shimmy and jerk, all of which are particularly desirable in wheelchairs used for transporting patients.
Unlike traditional nonholonomic vehicles, a holonomic vehicle can move in an arbitrary direction including sideways and/or rotate without changing the direction of the wheels. Therefore applications of holonomic vehicles include wheelchairs, which need to maneuver in crowded locations such as residential homes, hospitals and long-term care units as well as factories.
Holonomic vehicles employ special kinds of wheels including wheels having free rollers disposed about the main wheels. Any wheel capable of providing holonomic motion, alone or in appropriate combination with other wheels of the same or other type, will be referred to herein and in any appended claims as a "wheel providing holonomic motion." Some wheels providing holonomic motion generate horizontal vibrations which might be a significant problem in a wheelchair application. An omnidirectional vehicle described in copending U.S. application Ser. No. 08/840,522 uses spherical tires held by a novel ring roller mechanism that transmits an actuator torque to the ball wheel.
A wheelchair used in a complex indoor environment should meet several requirements. First, it is desirable that the vehicle body be compact enough to go through arrow doorways. Residential doors are limited in width; the vehicle's tread and chassis width must confirm to the dimensional constraints. The footprint of the wheelchair must be wide enough to prevent the patient from falling on the floor despite shift in the mass centroid of the patient body. A large footprint is therefore desirable for stability and safety, while wheelchairs must conform to dimensional constraints. Stability and maneuverability are therefore conflicting requirements. Traditional vehicle designs based on fixed footprint are inefficient in solving the stability-maneuverability trade-off problem.
Additionally, a four-wheeled vehicle is desirable to maintain static stability of the vehicle, but incurs the over constraint problem between the wheels and the ground because three degree-of-freedom (DOF) motion of the vehicle is controlled by four independent motors. The over constraint problem may result in slip at the wheels or generate unwanted internal forces within the vehicle chassis.